Chiral materials which change their chirality upon photoirradiation are known in prior art. For example, photoisomerizable chiral materials were reported which show E-Z or cis-trans isomerization upon photoirradiation and are thereby converted from one chiral form into another chiral form. Further known are photodegradable or (photo)tunable chiral materials (TCM) that change from chiral to achiral or to a racemic mixture upon photoirradiation, due to destruction of their chirality by photoelimination or photocleavage of the chiral center.
Photoisomerizable chiral materials have been suggested inter alia for the preparation of cholesteric polymer films with patterned optical properties, which can be used as optical components like colour filters or broadband reflective polarizers in liquid crystal displays. The preparation of patterned cholesteric films is described for example in WO 00/34808.
Furthermore, photoisomerizable and phototunable chiral materials have been suggested for use in cholesteric or multi-domain liquid crystal displays.
For example, WO 98/57223 discloses a multi-domain liquid crystal display with a nematic liquid crystal material comprising a polymerizable menthone derivative as photoisomerizable chiral dopant. The display comprises different sub-pixels in which the twist sense of the liquid crystal material is mutually opposite. It is manufactured by photoirradiation of a layer of liquid crystalline material containing a photoisomerizable chiral dopant with a given twist sense and a non-isomerizable chiral dopant with opposite twist sense through a photomask. This causes the isomerizable dopant in the exposed parts of the layer to change its chirality, leading to a change of the helical pitch in the exposed parts.
U.S. Pat. No. 5,668,614 discloses a multicolour cholesteric display made from a cholesteric liquid crystal mixture comprising a tunable chiral material (TCM). The display is prepared by partially exposing the liquid crystal mixture with the TCM to photoirradiation through a photomask. This leads to a change of the chirality of the TCM by photocleavage or photoracemisation and thus to a change of the helical pitch in the exposed parts of the cholesteric liquid crystal material. Thereby regions with different pitch and thus different colours of the reflected wavelength are obtained and a multicolour display is realized.
Photoisomerizable chiral materials comprising menthone, camphor or nopinone derivatives or chiral stilbenes have been reported by P. van de Witte et al., Liq. Cryst. 24 (1998), 819–27, J. Mat. Chem. 9 (1999), 2087–94 and Liq. Cryst. 27 (2000), 929–33 and A. Bobrovski et al., Liq. Cryst. 25 (1998), 679–687.
Tunable chiral materials (TCMs) comprising a photocleavable carboxylic acid group or aromatic keto group attached to the chiral center are disclosed in U.S. Pat. No. 5,668,614. Furthermore, F. Vicentini, J. Cho and L. Chien, Liq. Cryst. 24 (1998), 483–488 describe binaphthol derivatives as TCMs and their use in multicolour cholesteric displays.
However, the isomerizable and tunable chiral materials of prior art have several drawbacks. The TCMs reported in U.S. Pat. No. 5,668,614 and by F. Vicentini et al. have the general disadvantage that photocleavage is an irreversible process and leads to destruction of the chiral compound. The photoisomerizable menthone and stilbene derivatives disclosed in in WO 98/57223 and the articles of P. van de Witte et al. and A. Bobrovsky et al. have the disadvantage that they are not easily structurally modified due to a lack of functionality.
Another drawback of many photoisomerizable compounds known from prior art is that they exhibit only a low helical twisting power (HTP). The HTP describes the effectiveness of a chiral compound to induce a helically twisted molecular structure in a liquid crystal host material, and is given in first approximation, which is sufficient for most practical applications, by equation (1):
                    HTP        =                  1                      p            ·            c                                              (        1        )            wherein c is the concentration of the chiral compound and p is the helical pitch. As can be seen from equation (1), a short pitch can be achieved by using a high amount of the chiral compound or by using a chiral compound with a high absolute value of the HTP. Thus, in case chiral compounds with low HTP are used, high amounts are needed to induce a short pitch. This is disadvantageous, because chiral compounds often negatively affect the properties of the liquid crystalline host mixture, like for example the clearing point, the dielectric anisotropy Δε, the viscosity, the driving voltage or the switching times, and because chiral compounds can be used only as pure enantiomers and are therefore expensive and difficult to synthesize.
Another disadvantage of chiral compounds of prior art is that they often show low solubility in the liquid crystal host mixture, which leads to undesired crystallization at low temperatures. To overcome this disadvantage, typically two or more different chiral compounds have to be added to the host mixture. This implies higher costs and also requires additional effort for temperature compensation of the mixture, as the different chiral compounds usually have to be selected such that their temperature coefficients of the twist compensate each other.
Therefore, there is a considerable demand for chiral photoisomerizable compounds with a high HTP which are easy to synthesize in a large range of derivatives, can be used in low amounts, show improved temperature stability of the cholesteric pitch e.g. for utilizing a constant reflection wavelength, do not affect the properties of the liquid crystalline host mixture and show good solubility in the host mixture.
The invention has the aim of providing chiral photoisomerizable compounds having these properties, but not having the disadvantages of the chiral compounds of prior art as discussed above. Another aim of the invention is to extend the pool of chiral photoisomerizable compounds available to the expert.
It has been found that the above aims can be achieved by providing photoisomerizable chiral compounds according to claim 1.
Definition of Terms
The terms ‘liquid crystalline or mesogenic material’ or ‘liquid crystalline or mesogenic compound’ should denote materials or compounds comprising one or more rod-shaped, lath-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal phase behaviour. Rod-shaped and lath-shaped mesogenic groups are especially preferred. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerized.
For the sake of simplicity, the term ‘liquid crystal material’ is used hereinafter for both liquid crystal materials and mesogenic materials, and the term ‘mesogen’ is used for the mesogenic groups of the material.
The term ‘helically twisted structure’ refers to anisotropic materials, like for example liquid crystal materials, that exhibit a chiral mesophase wherein the mesogens are oriented with their main molecular axis twisted around a helix axis, like e.g. a chiral nematic (=cholesteric) or a chiral smectic phase. Materials exhibiting a cholesteric phase or chiral smectic C phase are preferred. Particularly preferred are materials exhibiting a cholesteric phase.
The term ‘film’ includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.
The term “photoisomerizable group” means a group that shows isomerization, for example cis-trans or E-Z isomerization, imparting a change in shape upon photoirradiation with a suitable wavelength, preferably in the range from 250 to 400 nm, very preferably from 300 to 400 nm.